The human body comes equipped with more than 650 muscles. These muscles, together with other connective tissues (joints, ligaments, and tendons), form the support system for the skeleton and allow the body to move. The three types of muscles are:

•  Skeletal muscles. These muscles manipulate the skeleton to cause movement. Skeletal muscles are voluntary muscles because their actions can be controlled.

•  Smooth muscles. These muscles are found in many places in the body, such as the bladder, arteries, veins, and the digestive tract. Smooth muscles are involuntary muscles because their actions occur without conscious control.

•  Cardiac muscle. This type of involuntary muscle is found only in the heart. Several layers of cardiac muscles make up the heart wall, which contracts and relaxes to keep the heart pumping.

Joints, ligaments, and tendons work with the different types of muscles to keep the body moving and working. Joints and tendons are surrounded by synovial tissue—a sheath that secretes a clear, thick lubricating fluid.

•  Joints. The main function of joints is to connect bone to other bone and to allow movement where it is needed. Some joints, such as those found in the skull, do not move at all; others, such as those found in the spine, have restricted movement. Joints in the arms, legs, and shoulders permit a great deal of movement.

•  Ligaments. Ligaments cross over joints and help join bone to bone. The ligaments support joints and help prevent dislocation.

•  Tendons. Tendons connect skeletal muscles to bones. These cords of tissue are flexible and very strong. They transfer force to the bones to cause movement. For example, the Achilles tendon connects the calf muscles to the bone in the heel and allows the foot to push against the ground when walking.